1. Field of the Invention
The present invention pertains generally to telecommunications, and particularly to a High Speed Downlink Packet Access (HSDPA) system such as that operated (for example) in a Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (UTRAN).
2. Related Art and Other Considerations
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based Wideband Code Division Multiple Access (WCDMA) access technology. As wireless Internet services have become popular, various services require higher data rates and higher capacity. Although UMTS has been designed to support multi-media wireless services, the maximum data rate is not enough to satisfy the required quality of services. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
One result of the forum's work is the High Speed Downlink Packet Access (HSDPA). The HSDPA system provides, e.g., a maximum data rate on the order of about 10 Mbps to improve the radio capacity in the downlink. HSDPA features a high speed channel (HSC) controller that functions, e.g., as a high speed scheduler by multiplexing user information for transmission over the entire HS-DSCH bandwidth in time-multiplexed intervals (called transmission time intervals (TTI)). Since HSDPA uses code multiplexing, several users can be scheduled at the same time.
HSDPA achieves higher data speeds by shifting some of the radio resource coordination and management responsibilities to the base station from the radio network controller. Those responsibilities include one or more of the following (each briefly described below): shared channel transmission, higher order modulation, link adaptation, radio channel dependent scheduling, and hybrid-ARQ with soft combining.
In shared channel transmission, radio resources are shared between users using time multiplexing. A high speed-downlink shared channel is one example of shared channel transmission. One significant benefit of shared channel transmission is more efficient utilization of available code resources as compared to dedicated channels (DCH). Higher data rates may also be attained using higher order modulation, which is more bandwidth efficient than lower order modulation, when channel conditions are favorable.
Radio channel conditions experienced on different communication links typically vary significantly, both in time and between different positions in the cell. In traditional WCDMA systems, power control compensates for differences in variations in instantaneous radio channel conditions. With this type of power control, a larger part of the total available cell power may be allocated to communication links with bad channel conditions to ensure quality of service to all communication links. But radio resources are more efficiently utilized when allocated to communication links with good channel conditions. For services that do not require a specific data rate, such as many best effort services, rate control or adjustment can be used to ensure there is sufficient energy received per information bit for all communication links as an alternative to power control. By adjusting the channel coding rate and/or adjusting the modulation scheme, the data rate can be adjusted to compensate for variations and differences in instantaneous channel conditions.
For maximum cell throughput, radio resources may be scheduled to the communication link having the best instantaneous channel condition. Rapid channel dependent scheduling performed at the base station allows for very high data rates at each scheduling instance and thus maximizes overall system throughput. Hybrid ARQ with soft combining increases the effective received signal-to-interference ratio for each transmission and thus increases the probability for correct decoding of retransmissions compared to conventional ARQ. Greater efficiency in ARQ increases the effective throughput over a shared channel.
With HSDPA, the physical layer becomes more complex as an additional MAC protocol is introduced: the MAC-hs. On the network side, the MAC-hs protocol is implemented in the radio base station (RBS). The MAC-hs protocol contains the retransmission protocol, link adaptation, and channel dependent scheduling. The complexity increase with HSDPA is thus mainly related to the introduction of an intelligent Layer 2 protocol in the radio base station (RBS).
HSDPA generally has an algorithm for selecting the amount of power for the HS-DSCH and a downlink control channel known as the HS-SCCH. The HS-SCCH contains information which is sent to the mobile terminals so that the mobile terminals know if they have data to receive on the HS-PDSCh channel or not.
As mentioned above, HSDPA uses a fast Hybrid ARQ between a radio base station (RBS) and a mobile station (UE). Iub Flow Control is used between a radio network controller (RNC) and the RBS in order to ensure that a feasible amount of data (bits or bytes) is in the RBS priority queues. The amount of data in the queues is controlled by a flow control algorithm (FCA). If the RBS priority queues (PQs) are too lengthy these buffers (PQs) give too long delays. On the other hand, if the RBS priority queues (buffers) are too short, they become “dry” at a time when a user is suddenly scheduled in the transmission interval. Thus, the flow control algorithm (FCA) seeks to control the RBS priority queue flows in a way that the RBS priority queues become essentially stable.
The data in the priority queues is sent from a control node to a radio base station in protocol data units (PDUs). A number of PDUs may be included in each high-speed downlink shared channel (HS-DSCH) data frame.
In a HSDPA system there are generally two basic bottlenecks. One of the bottlenecks is on the downlink on the air-interface (Uu) between the mobile station and the radio base station node (RBS); the other bottleneck is on the downlink on the interface (Iub) between the radio base station node and the radio network controller node. Both of these bottlenecks are considered in the flow control algorithm. The available HS bandwidth over the Iub interface varies considerably. If too much HS traffic is allocated over Iub, frame losses and long delays degrades the HS packet data performance. The air interface scheduling of HS-DSCH Data Frames is controlled by RBS.
When the radio conditions for a HS-DSCH user begins to be poor in the source cell, the user is connected to a new cell (target cell) with better radio connection. Such can occur, for example, as a mobile station travels farther and farther from a radio base station which broadcasts in a “source” cell in which the mobile station has been receiving and sending transmissions, and approaches a new or “target” cell in which another radio base station is operative. The target cell may be served by the same or another radio base station. Since, from the perspective of the traveling mobile station, the radio conditions of the source cell decline and the radio conditions of the target cell improve, the radio access network must provide some means of handoff or handover of the mobile station from the source cell to the target cell, e.g., a cell change for the mobile station. On the downlink the cell change is necessary so that further data frames which continue the stream or media previously sent via the source cell can be sent via the target cell to the mobile station. In the specific case of a HSDPA cell change, HS-DSCH data frames are switched over from being sent to source cell to being sent to target cell.
The HSDPA cell change is a GSM-like hard handover. That is, at a given point of time (e.g., “Activation Time”) the source cell RBS stops its transmission to the mobile station and the new (target) cell starts its transmissions to the mobile station. Soft Handover (SHO), which is generally employed for wideband CDMA (WCDMA) cannot be used for HS-DSCH. At a “switch time” which occurs before the activation time the radio controller node (e.g., control node) must stop sending high-speed downlink shared channel (HS-DSCH) data frames to the source radio base station for the source cell and instead reallocate the high-speed downlink shared channel (HS-DSCH) data frames to the target radio base station for the target cell.
During the HSDPA cell change procedure the data transmission speed might degrade for various reasons. A first reason is that data directed to the source cell may be lost at call change activation time (e.g., when the cell change takes effect on the air interface). Such can occur, for example, if in the source cell some protocol data units (PDUs) cannot be sent to the mobile station before the time of the cell change, with the result that these PDUs will be lost and must be retransmitted by RLC protocol. Another reason is that the target cell may not have PDUs to send to the mobile station. The challenge is to stop sending data for the mobile station via the source cell at an appropriate time (at the latest at the switch time) and to start sending data to the mobile station via the target cell in order to fill up the priority queue with an appropriate amount of data just before activation time.
What is needed, therefore, and an object herein provided, are apparatus, method(s), and technique(s) for providing HSDPA flow control in a way to minimize data transmission speed degradation during a HSDPA cell change.